It has been proposed a method of forming a seed crystal of gallium nitride on an orientated polycrystalline substrate and then forming a thick film of gallium nitride, so that a self-supporting substrate of polycrystalline gallium nitride composed of a plurality of gallium nitride-based single crystal grains is produced, each having a single crystal structure substantially in a direction of normal lime (Patent documents 1 and 2).
Further, in producing a crystal of a nitride of a group 13 element by a flux method in a melt, it is proposed to generate the melt by heating a raw material of a group 13 element, a material of at least one of an alkali metal and an alkaline earth metal, and a liquid material of germanium (Patent document 3).